In the recent past, a significant advancement has been achieved in the development of mobile radios and mobile telephones. As a result, all types of vehicles can be equipped with mobile communication equipment to allow persons to remain in voice or data contact with others when traveling from one location to a different location. In the early development of mobile communication equipment, mobile radios, and the like, were adapted for communicating with a single station (e.g., cell or site), as that system was often the only system in the particular geographical area of operation. A mobile radio station of this type could operate on a number of different communication channels, where each channel was associated with a different transmit and receive frequency in the 800-900 megahertz spectrum.
Initially, the radio channels of a system were dedicated to particular users and could not be used, even when idle, with other users. The trunking of mobile radio channels was developed so that each user of a system could communicate on any of the channels, if a channel was idle when communications were desired. While the trunking of communication channels greatly facilitated the efficiency and the use of communications, the technique created other problems. For example, in a trunked radio system, a mobile radio is required to scan the various channels to determine if an incoming call is directed to that particular radio. Further developments in the mobile radio field led to the use of a "home" channel that was dedicated for transmitting data messages to the various mobile radios to signal when an incoming call is designated to a particular mobile radio. In this type of arrangement, when the mobile radios were turned on and in an idle mode, the radio would automatically monitor the home channel for data messages to determine if any incoming call was directed to that particular mobile radio. The different mobile radios are distinguished from each other in this type of system by assigning a unique identification code to each mobile radio. The data message transmitted on the home channel by the central system would include the particular identification code assigned to a particular radio if an incoming call was intended to be completed to that mobile radio. Hence, each mobile radio had to monitor the home channel, retrieve all of the identification codes and compare them with the unique identification code assigned to the radio. If a match was found, then the mobile radio would automatically respond, and the system controller would switch from the home channel to an idle voice communication channel, whereby voice communications could be carried out.
As the demand for mobile communications increased, the number of central systems also increased. Presently, there may be numerous systems in a single locale of operation. Generally, each mobile radio is allowed to operate with a particular system by way of license or other contractual arrangement. Further, each system controller has programmed therein the identification code of the system itself, as well as the codes of the various radios allowed to operate therewith. In situations where adjacent systems do not overlap in terms of area of transmission coverage, the systems can operate with the same channel frequencies. On the other hand, if there is an overlapping coverage between adjacent systems, the channel frequencies cannot be the same as there would be interference between the same channels.
The use of multiple stations in an area of operation greatly facilitates the area by which the mobile radio user can communicate, however, other problems are presented. For example, a mobile radio operating in a multi-system area must scan the various systems, and particularly the home channel of each authorized system and compare the various identification codes to determine if an incoming call is intended for the mobile radio. The data message in which the code is embedded is generally about 134 milliseconds long. The number of systems by which a mobile radio can operate is generally limited by the circuits of the mobile radio itself, and is often limited to 10 systems and 10 groups within each system. The scanning by a mobile radio between different systems has traditionally been carried out in a sequential manner starting with system 1 and proceeding through system 10, and then repeating the sequence. The switching between the home channel of one system and the home channel of another system is generally accomplished by use of a frequency synthesizer in the mobile radio. The change of frequency and the detection of the carrier in a mobile radio takes a certain amount of time. Further, the mobile radio must detect and receive the data message on the selected home channel, and usually detect two data messages to ascertain with a high degree of certainty that the data message is valid. Next, the mobile radio must decode and match the identification code received in the data message of the home channel with the unique identification code programmed into the mobile radio. If a match is not found, then the mobile radio must reprogram the frequency synthesizer, proceed to the next home channel of the next system in the sequence. Conventional mobile radios generally take about 1/4-1/2 second in order to scan and check each system home channel to determine if an ID code match exists. It can be appreciated that when a number of systems are to be scanned, a substantial amount of time is required. Further, the longer the time between scans of a particular system, the higher probability that the mobile radio will not presently be scanning a home channel in which there is transmitted the identification code of the mobile radio itself.
Once communications are established between a mobile radio and an originating destination, the actual communications occur in a piecemeal manner, in that there are pauses and gaps between the voice communications of the two parties. In these pauses and gaps between the voice communication, the channel can be seized and used for communications between two other parties. Thus, the foregoing scanning sequence may be required to be carried out numerous times for seizing a communication channel during a single conversation if a programmed dwell time is exceeded. Further, if the routine communications generally occur on systems that appear late in the scanning sequence, time is unnecessarily taken to scan the stations early in the sequence in order to access the stations where there is a higher likelihood of establishing communications.
While the trunking of communication channels has greatly facilitated the use by many users of a few channels, substantial demands are placed on the scanning routine of mobile radios and thus efforts are constantly being made to decrease the scanning time and thus reduce the time intervals for detecting an incoming communication and seizing an available communication channel. Provisions are available in mobile telephone equipment to allow roaming outside the normal cells of operation and yet remain available for receiving incoming calls. This feature is carried out by transferring information to the remote cell where the mobile telephone will be operated. Thus, when an incoming call is generated in the locale of normal operation, it will be transferred to the remote cell and received by the mobile telephone. The "roaming" feature has not yet been established with mobile radios, but such a communication system would be facilitated with the roaming feature. However, both mobile radios and telephones would be enhanced if the roaming feature could be carried out adaptively and transparent to the user so that no overt action is required to receive communications in remote areas of operation.
In view of the foregoing, it can be seen that a need exists for a technique that reduces the scanning time, and thus increases the possibility of achieving a match of ID codes and the seizing of an idle transmission channel. A further need exists for a technique that modifies the conventional scanning sequence so that it is weighted more heavily to increase the likelihood of success early in the scanning sequence. A further need exists for allowing roaming of mobile communication equipment and the automatic "finding" of the mobile equipment and the transferral of incoming communications thereto.